The two-step aging behavior of Al-7%Si-0.3 or 0.75%Mg casting alloys was investigated by means of the mechanical properties, electrical resistivity and transmission electron microscopy. The alloys were solution-treated at 535℃, quenched, preaged at -55 to 125℃ for 23hr and finally aged at 140 or 180℃ for 5hr. Preaging at -55℃ has no effect on the properties obtained after the final aging. The hardness, tensile and yield strength of the alloys preaged at 25℃ is lowest in all the alloys preaged at -55 to 125℃. If preaged at 125℃, the alloy has mechanical properties identical to those of the alloy without preaging. The positive effect of two-step aging is never found in these casting alloys. These facts are discussed on the bases of change in supersaturation caused by cluster formed during preaging and impossibility of clusters to transform into G.P. zone during final aging.